Voltage regulator

ABSTRACT

A low dropout voltage regulator comprising a series-regulating element (T 1 ) between an input (I) and an output (O) of the voltage regulator, and a differential input error amplifier ( 1 ) having a first output (O 1 ) coupled to a control input of the series-regulating element (T 1 ), characterized in that the error amplifier ( 1 ) further comprises a second output (O 2 ) coupled to the output (O) via a high-pass filter ( 5,  C 1,  R 1 ).

The invention relates to a voltage regulator comprising aseries-regulating element between an input and an output of the voltageregulator, and a differential input error amplifier having a firstoutput coupled to a control input of the series-regulating element.

Low dropout voltage regulators are widely used building blocks in almostany electronic application. They adapt an external power supply to theneeds of the supplied circuit. In portable applications as in mobilephones a main requirement for the voltage regulator is a low dropoutvoltage and a good stability over a large range of capacitive loads.

In applications requiring read/write operations on an optical disk asCD, DVD, Blue-ray Disk (BD), a photo diode integrated circuit (PDIC) isprovided. In such an IC, the photo diode detector, supplied from a photodiode supply filter, is followed by a programmable amplifier. In priorart Photo Diode ICs, the photo diode supply filter comprises a passivefirst order RC low-pass filter as shown in FIG. 7. The resistor R isadapted to the current supply demands of the photo diode and therefore,depends on the received laser light intensity. To keep a voltage dropacross resistor R sufficiently low, the resistor value is adapteddepending on current requirement (light intensity). The currentrequirement is approximately inversely proportional with the gain of apre-amplifier. Consequently, the cut-off frequency of the filterf._(3dB)=1/(2πRC) changes depending on the current requirement. At lowfrequencies (far below f._(3dB)) the output impedance of the filter isdetermined by R.

High intensity light pulses are used to write on an optical disc (CD,DVD or BD (Blu-ray Disc)). The PDIC is used to monitor the writeprocess. Directly following a high intensity ‘Write’ laser pulse, a lowintensity read out is needed, see FIG. 6. A write level WL of the photocurrent PC during write period W is about 80x a read level RL duringread period R. The low intensity read outs contain mostly servo controlinformation, including a wobble signal. The PDIC photo diode andamplifiers have to recover from the write pulse fast enough to providesufficient accuracy in subsequent read operation. This settling time isgoverned by bandwidth and flatness of response as function of frequency,both in terms of amplitude and phase. These requirements also translateto the output impedance of the supply filter. The simple filter shown inFIG. 7 is not fit for use in read/write operations with such stringentsettling requirements. Based on FIG. 6 it can be derived that for thefilter shown in FIG. 7 having a capacitor of 40 pF, a resistance of theresistor should not exceed 60 ω. However, with these values the requiredfilter suppression associated with the high gain settings are no longermet.

U.S. Pat. No. 6,373,233 describes a low dropout voltage regulator withimproved stability for all capacitive loads. The low dropout voltageregulator comprises a series controlled p-MOS transistor controlled by adifferential input error amplifier. An output of the error amplifier iscoupled to an output terminal of the series controlled p-MOS transistorvia a series connection between a capacitor and a resistor. A feedbackfrom the output of the low dropout voltage regulator to an input of theerror amplifier is provided, too. The series controlled p-MOS transistoracts as an integrator and consumes a too large amount of phase margin toallow low output impedance also at high frequencies. Hence, the low dropvoltage regulator presented in that prior art patent cannot provide lowoutput impedance at high frequencies and therefore it is not suitable inapplications involving read/write processes as CD, DVD and BD.

It is therefore an object of the present invention to provide animproved voltage regulator.

In accordance with the invention this is achieved in a voltage regulatoras described in the first paragraph, characterized in that the erroramplifier further comprises a second output coupled to the output via ahigh-pass filter. DC and low frequencies are filtered by theseries-regulating element while relatively high frequencies are filteredby the high-pass filter. Both branches are controlled in parallel by thedifferential input error amplifier. The circuit allows a relativelylower voltage drop between the input signal and the output signal.

In an embodiment of the invention a first low-pass filter is coupledbetween input and an input terminal of the series-regulating element. Itis experimentally determined that the first low-pass filter is stilladvantageous for obtaining good overall filter suppression.

The first low-pass filter may comprise a first plurality of resistorsconnected in series, the first plurality of resistors being coupled to afirst plurality of respective switches for modifying a time constant ofthe first low-pass filter. A time constant of a first order low-passfilter is proportional to a product between the resistor value and thecapacitor value of the filter. Modifying the value of the resistor inthe filter results in a modification of the time constant. A cut-offfrequency of the filter is inverse proportional with the product betweenthe capacitor and the resistor values and therefore, the cut-offfrequency is also changed when the resistor value is changed.

In an embodiment of the invention a first input of the error amplifieris coupled to the input of the regulator through a second low-passfilter. The second low-pass filter may comprise a series coupling of adrop voltage source, a second resistor and a second capacitor. Thereference of the error amplifier is the supply voltage, reduced with asmall DC voltage and low-pass filtered by the second capacitor and thesecond resistor. The drop voltage could be obtained as e.g. a bipolarjunction transistor connected as a diode or as a fixed bias currentthrough a resistor. It could be pointed out here that the referencevoltage could be also a supply-voltage independent voltage source as instabilization circuits.

In another embodiment of the invention the series-regulating elementcomprises a plurality of series-regulating elements coupled to arespective second plurality of selectable resistors. The secondplurality of selectable resistors is implemented as field effecttransistors coupled in pairs. Each pair comprises a series connection ofmain current channels of two transistors coupled between the input andan output of the first amplifier or between an output of the firstlow-pass filter and the output of the first amplifier. When a selectableresistor pair is selected the output of the low dropout voltageregulator is inputted to a specific selectable series-regulating elementfor adapting to load requirements for the low dropout voltage regulator.

In another embodiment of the invention the low dropout voltage regulatoris used in an optical detector/amplifier for supplying one or more photodiodes coupled to variable gain amplifiers. Having a relative fastsettling time and a relative large bandwidth, the low dropout voltageregulator according to the invention is a better solution forapplications in which a photo diode receives an optical signal from anoptical data carrier. The variable gain amplifier preferably comprises aplurality of cascaded-connected controllable amplifiers for obtaining asufficient amplification of the signal generated by the photo diode.

The above and other features and advantages of the invention will beapparent from the following description of the exemplary embodiments ofthe invention with reference to the accompanying drawings, in which:

FIG. 1 depicts an embodiment of a low dropout voltage regulator having alow output impedance according to the invention,

FIG. 2 depicts an implementation of a first low-pass filter, accordingto an embodiment of the invention,

FIG. 3 depicts an implementation of a second low-pass filter, accordingto an embodiment of the invention,

FIG. 4 depicts an implementation of a selectable series-regulatingelement, according to an embodiment of the invention,

FIG. 5 depicts an implementation of an optical detector/amplifier,according to an embodiment of the invention,

FIG. 6 depicts a typical dependency between the photo—current of thePDIC and the WRITE/READ process,

FIG. 7 depicts a prior art implementation of the first low-pass filter,and

FIG. 8 depicts a typical characteristic capacity versus reverse voltageof a photo diode used in an optical detector/amplifier according to oneembodiment of the invention.

FIG. 1 depicts an embodiment of a low dropout voltage regulator 100according to the invention. The voltage regulator 100 comprises aseries-regulating element T1 between an input I and an output O of thevoltage regulator. The low dropout voltage regulator 100 includes adifferential input error amplifier 1 having a first output O1 coupled toa control input of the series-regulating element T1 via a firstamplifier 4. The first amplifier A1 may have an amplification factorof 1. A series connection of a capacitor Cn1 and a resistor Rn1 arepreferably coupled between the first output O1 of the error amplifierand the output O of the voltage regulator to increase stability. Thecapacitor Cn1 may have a capacitance value of 2 pF. A capacitor C4 and abias current source Ibias are coupled between the output O of the erroramplifier and ground. The capacitor C4 may have a capacitance of 80 pF.The error amplifier 1 further comprises a second output O2 coupled tothe output O via a high-pass filter. In this embodiment, the high-passfilter comprises a second amplifier 5, which may have an amplificationfactor of 1, coupled to a series combination of a first capacitor C1 anda first resistor R1. The resistor R1 may not be necessary. Theamplifiers 4 and 5 may be left out if output stages of the erroramplifier 1 are designed suitably.

The embodiment of FIG. 1 is a ‘split band’ structure with a PMOST seriestransistor T1 driven by amplifier A1, supported by a capacitive branchC2 driven by amplifier A2. DC and low frequencies are taken care off byT1 while class AB amplifier A2 via C2 takes care of the highfrequencies. Both branches are controlled in parallel from erroramplifier 1. The structure allows lowest voltage drop between input andoutput, governed by T1. As set out below with reference to FIG. 3, thereference to the error amplifier 1 is the supply voltage, reduced with asmall DC voltage ‘Vdrop’ and filtered by a low-pass filter consisting ofR1 plus C1. Since gain of the PMOST transistor T1 highly depends onDrain-Source DC current, a fixed bias Ibias ensures a certain minimumgain also with no light on the photo diodes. Empirically, it was foundthat a pre-filter 3 (that may be as simple as in FIG. 7) is stilladvantageous to obtain good overall filter suppression. Like the priorart solution, this simple implementation of the low-pass filter 3 uses aresistor R that is adapted in value in accordance with the currentrequirement. The available voltage drop ‘Vdrop’ is shared equal parts bypre-filter 3 and PMOST T1.

Passive filters normally used in low dropout voltage regulators are notfit for use in read/write applications having stringent requirements. Anactive filter as in U.S. Pat. No. 5,434,535 could also be considered. Inthat prior art patent a differential amplifier is used that has afeedback capacitor, the capacity of the capacitor being magnified by anamplification of the amplifier. Providing a correct biasing for theamplifier, the filter works properly for filtering relatively highfrequency signals but is limited at relative low frequency by the outputvoltage excursion capability of the amplifier.

The applications requiring read/write operations on an optical diskcould be CD, DVD, Blue-ray Disk (BD). In these applications aphoto-detector integrated circuit (PDIC) is provided. The PDIC is usedfor monitoring a write process. Directly following a high intensitywrite laser pulse, a low intensity read out is necessary as it is shownin FIG. 6. The low intensity read outs contain mostly servo-controlinformation, including a wobble signal. The PDIC photo diode has torecover from the write pulse fast enough for providing a sufficientaccuracy in subsequent read operation. The settling time, amplitude andphase are determined by bandwidth and flatness response versusfrequency. These requirements determine restrictions for the magnitudeof the output impedance of the filter. Using a passive filter as shownin FIG. 7 in the above-mentioned conditions, the filter suppressionrequirements associated to high gain settings conditions are no longermet. In order to comply with these requirements the low dropout voltageregulator shown in FIG. 1 could be used. Hence, a first low-pass filter3 coupled between the input I and the main current channel of theselectable series-regulating element T1, is used. DC and low frequencysignals are filtered by the series-regulating element T1 controlled bythe first amplifier 4. The class AB second amplifier 5 coupled in seriesto the first capacitor C1 and the first resistor R1 filter highfrequency components. The circuit allows a relatively lower voltage dropbetween the input I signal Vcc and the output O signal VOUT.

An input+of the error amplifier 1 is coupled to a second low-pass filter2, the second low-pass filter 2 being connected between the input I anda reference terminal. In an alternative embodiment, the second low-passfilter is absent, and the+input of the error amplifier 1 is coupled to areference voltage source.

FIG. 2 depicts a preferred implementation of the first low-pass filterLPF 1, according to an embodiment of the invention. The first low-passfilter 3 comprises a first plurality of resistors R2 a, R2 b, R2 cconnected in series coupled to a first plurality of respective switchesT2 a, T2 b, T2 c for modifying a time constant of the first low-passfilter 3. In FIG. 2 the switches are represented as P-MOS transistorsbut a skilled person in the art could also imagine the use of otherswitching elements as e.g. N-MOS transistors, phototransistors orpassive switches. Furthermore, control signals S2 a, S2 b and S2 c areconsidered to be voltages but currents and light could be also be usedand a skilled person in the art could easily derive a suitable circuit.When a switch T2 a, b or c is in an ON state, its equivalent resistancein relatively low, much lower than a value of the resistance of anyresistor R2 a, b or c. When a switch T2 a, b or c is in an OFF state,it's equivalent resistance in relatively big, much bigger that a valueof the resistance of any resistor R2 a, b or c. It results that when allthe switches are in an OFF state the equivalent resistance of thecircuit is R2 a+R2 b+R2 c+R2 d and a time constant of the filter is (R2a+R2 b+R2 c+R2 d)*C2. If T2 a is ON the equivalent resistance of thecircuit is R2 b+R2 c+R2 d and, consequently, the time constant of thefilter is (R2 b+R2 c+R2 d)*C2. Similar reasoning could be used for anycombination of ON state switch—OFF state switch. Hence, the cut-offfrequency of the low-pass filter could be adapted taking into accountthe technical constraints of an application, allowing more flexibilityin applications.

FIG. 3 depicts an implementation of a second low-pass filter LPF2,according to an embodiment of the invention. The second low-pass filter2 comprises a series coupling of a voltage source Vdrop, a secondresistor R3 and a second capacitor C3 between the input In and areference node Ref. The second low-pass filter delivers a referencesignal to the error amplifier 1. The reference signal is obtained aftera low-pass filtering of the signal Vcc inputted to the input I of thelow dropout voltage regulator 100 after reducing it with a relativelysmall voltage Vdrop. The voltage Vdrop could be obtained using e.g. abipolar junction transistor connected as a diode or as a fixed biascurrent through a resistor. The reduced signal is fuirther low-passfiltered by the series combination of the second resistor R3 and secondcapacitor C3. The voltage on the second capacitor C3 is inputted to theerror amplifier 1 as reference voltage.

FIG. 4 depicts an implementation of a selectable series-regulatingelement T1, according to an embodiment of the invention. The selectableseries-regulating element T1 comprises a plurality of series-regulatingelements T1 a, T1 b, T1 c coupled to a respective second plurality ofselectable resistors R11 a, R12 a, R11 b, R12 b, R11 c, R12 c. Thesecond plurality of selectable resistors R11 a, R12 a, R11 b, R12 b, R11c, R12 c are P-MOS transistors coupled in pairs. Each pair comprises aseries connection of a main current channels of two transistors coupledeither between the input I and an output of the first amplifier 4 orbetween an output of the first low-pass filter 3 and the output of thefirst amplifier 4. It is understood that the controllable resistorscould be also N-MOS transistors or phototransistors. Control signals S1a, S1 b and S1 c are considered voltages but skilled persons in the artcould also imagine current signals or light signals. The selectableresistors R11 a, R12 a, R11 b, R12 b, R11 c, R12 c have a highresistance state and a low resistance state. When the resistors are in ahigh resistance state the respective series-regulating transistor T1 a,b or c does not conduct a current through it's main current channel.Actually, a very low leakage current circulates through the main channelof the transistors. When the resistors are in a low resistance state therespective series-regulating transistor T1 a, b or c does conduct acurrent through it's main current channel, supplying an output currentIbias.

FIG. 5 depicts an implementation of an optical detector/amplifier 200,according to an embodiment of the invention. The opticaldetector/amplifier 200 comprises a low dropout voltage regulator 100 forsupplying one or more photo diodes 201 coupled to a variable gainamplifier 202. The photo diode(s) 201 transmits a signal correspondingto a read or a write operation on or from an optical disc. The variablegain amplifier comprises a plurality of cascaded-connected controllableamplifiers. The low dropout voltage regulator 100 has to supply thephoto-diode(s) 201, the photo-diode(s) 201 receiving an optical signalgenerated by e.g. laser. The dependence of the photo—current in thephoto-diode(s) 201 versus the optical signal is shown in FIG. 6 and atypical characteristic capacity versus reverse voltage of a photodiode(s) 201 used in an optical detector/amplifier is shown in FIG. 8.The low dropout voltage regulator 100 according to this invention issuitable to be used in applications using this kind of diodes andsignals associated to as it was previously shown. An output signal ofthe photo-diode(s) 201 is amplified in a variable gain amplifier 202that is necessary to adapt the signal to a load. Depending on a specificapplication, there could be one amplifier 202 or a plurality ofamplifiers 202.

In summary, a low power, low dropout supply filter exhibiting low noiseand low output impedance is created. Operated in closed loop the PMOSTseries transistor T1 supplies output power for DC and low frequencies,while the capacitively coupled class AB amplifier output stage 5provides power for the high frequencies (up to 200 MHz). The fixedoutput parallel capacitor C4 takes over from there. By proper choice ofcomponents a smooth transition of operation over the three frequencyregions is obtained. The circuit is used in PDIC amongst others toreduce cross-talk and to meet stringent settling requirements.

It is remarked that the scope of protection of the invention is notrestricted to the embodiments described herein. Neither is the scope ofprotection of the invention restricted by the reference numerals in theclaims. The word ‘comprising’ does not exclude other parts than thosementioned in the claims. The word ‘a(n)’ preceding an element does notexclude a plurality of those elements. Means forming part of theinvention may both be implemented in the form of dedicated hardware orin the form of a programmed purpose processor. The invention resides ineach new feature or combination of features.

1. A voltage regulator comprising a series-regulating element (T1)between an input (I) and an output (O) of the voltage regulator, and adifferential input error amplifier (1) having a first output (O1)coupled to a control input of the series-regulating element (T1),characterized in that the error amplifier (1) further comprises a secondoutput (O2) coupled to the output (O) via a high-pass filter (5, R1,C1).
 2. A voltage regulator as claimed in claim 1, wherein a firstlow-pass filter (3) is coupled between the input (I) of the voltageregulator and an input terminal of the series regulating element (T1).3. A voltage regulator as claimed in claim 2, wherein the first low-passfilter (3) comprises a first plurality of resistors (R2 a, R2 b, R2 c)connected in series, the first plurality of resistors (R2 a, R2 b, R2 c)being coupled to a first plurality of respective switches (T2 a, T2 b,T2 c) for modifying a time constant of the first low-pass filter (3). 4.A voltage regulator as claimed in claim 1, wherein a first input (+) ofthe error amplifier (1) is coupled to the input (I) of the voltageregulator through a second low-pass filter (2).
 5. A voltage regulatoras claimed in claim 4, wherein the second low-pass filter (2) comprisesa series coupling of a voltage source (Vdrop), a resistor (R3) and acapacitor (C3).
 6. A voltage regulator as claimed in claim 1, whereinthe series-regulating element (T1) comprises a plurality ofseries-regulating elements (T1 a, T1 b, T1 c) coupled to a respectivesecond plurality of selectable resistors (R11 a, R12 a, R11 b, R12 b,R11 c, R12 c).
 7. A voltage regulator as claimed in claim 6, wherein thesecond plurality of selectable resistors (R11 a, R12 a, R11 b, R12 b,R11 c, R12 c) are field effect transistors coupled in pairs, each paircomprising a series connection of a main current channels of twotransistors coupled between the input (I) of the voltage regulator andthe first output (O1) of the error amplifier (1).
 8. An opticaldetector/amplifier (200) comprising a voltage regulator as claimed inany of the preceding claims for supplying one ore more photo diodes(201) coupled to a variable gain amplifier (202).
 9. An opticaldetector/amplifier (200) as claimed in claim 8, wherein the variablegain amplifier comprises a plurality of cascaded-connected controllableamplifiers.